Most of the energy consumed in the world today is “stored solar energy” in the form of fossil fuels, such as petroleum, natural gas, and coal. Fossil fuels, however, are finite and their combustion has been tied to an increase in the amount of carbon dioxide in the atmosphere and other pollutants in the environment. Their limited availability also has national security and economic implications.
Solar and nuclear energy are not limited in the same manner as fossil fuels. They can provide viable long-term persistent energy options and be an environmentally advantageous, long-term alternative to fossil fuels. Such sources can produce hydrogen from water, which can be used as an independent, clean-burning fuel.
Thermochemical processes for converting solar or nuclear energy into fuels are potentially more straightforward, efficient, and less costly than using electric power to electrolyze water. Thermochemical cycles utilize high-temperature heat and a series of chemical reactions to produce fuels. Thermochemical water-splitting cycles utilize a series of chemical reactions with the overall reaction H2O→H2+½O2. Thermochemical carbon dioxide-splitting cycles utilize a similar series of chemical reactions with the overall reaction CO2→CO+½O2. All of the other chemicals are recycled within the process.
Recent solar thermochemical research has focused on two-step metal oxide cycles that alternately thermally reduce a metal oxide, such as magnetite (Fe3O4) to wustite (FeO), producing oxygen, and then oxidize the metal oxide with water or carbon dioxide to produce hydrogen or carbon monoxide, respectively. The metal oxide is typically cycled between the high temperature thermal reduction step and a lower temperature re-oxidation step. For example, cerium oxide is a metal oxide that has received notable attention. The metal oxide cycles are attractive in that they involve only two chemical steps. However, they require temperatures of at least 1400° C. for reasonable efficiencies, even with the addition of other dopants to the metal oxides to lower the required thermal reduction temperature. These high temperatures preclude the use of nuclear power and severely impact the design and efficiency of solar collection hardware.
To split water or carbon dioxide at lower temperatures, known nuclear driven thermochemical cycle work has centered on cycles that involve the decomposition of sulfuric acid, with the sulfur-iodine and hybrid sulfur processes receiving significant attention. A number of thermochemical cycles have been proposed; however, they rely upon sulfur oxides or other corrosive or hazardous materials, require substantial amounts of electrical power, have failed to be operational, produce corrosive and/or toxic chemicals, are harmful to the environment, create safety concerns, or a combination thereof.
A system and process for producing fuel with a methane thermochemical cycle that do not suffer from one or more of the above drawbacks would be desirable in the art.